The technology widely used for the production in large scale of foamed polyester articles uses as starting material a resin grade which has already intrinsic properties for being foamed, and in that grade, the resin is sold to the producers of foamed articles.
In the patent literature there are described polymeric masterbatches suitable for the production of foamed polyester articles using as starting polyester resin standard bottle grade PET having intrinsic viscosity of about 0.8-0.9 dl/g. None of said masterbatches have, up to now, found industrial scale application due to some drawbacks so far unremoved.
One of said masterbatches is described in EP 0636158; it is obtained by distributing a dianhydride, such as pyromelitic dianhydride (PMDA) in molten PET wherein, during the preparation, some PET branching and some gel formation take place. Sublimation of PMDA also occurs at the extruder die due to the high processing temperature needed to compound the ingredients. The above problems lead to instability in the foaming process with the production of an uneven foam quality. The maximum quantity of PMDA usable is not higher than about 12% of the total masterbatch weight.
Another type of masterbatch is described in U.S. Pat. No. 5,810,206. It is obtained by distributing the PMDA in molten polyolefin at a temperature relatively high at which and at a temperatures subsequently needed in the foaming process ( temperatures as high as 280-300° C.), the polyolefin tends to degrade. Stabilizers are needed to prevent polyolefin degradation. This masterbatch has limitation in the poor PMDA dispersion and in the high amount of polyolefin added to the foamable mixture, which impairs some of the peculiar characteristics of a foamed polyester article, such as the thermal and dimensional stability. Moreover the masterbatch cannot be easily dried prior its use due to sticking problems caused by the low softening point of the polyolefins, which further reduce the efficiency of PMDA in the foaming process.